1. Field of the Invention
In the plastics industry, screws and cylinders are used in various injection molding processes to produce plastic articles. This invention relates to a high strength bimetallic cylinder used in these processes.
2. Description of the Related Art
In an injection molding process, solid plastic resin is heated and liquefied inside a hollow cylinder by heater bands that envelop the cylinder. The molten plastic is pressurized and conveyed into a heated zone inside the cylinder by a rotating screw. This heated zone is ahead of a check ring on the head of the screw and has a definite volume predetermined according to the plastic article to be made. Then a forward movement of the screw and check ring inside the cylinder injects this volume of plastic into a mold cavity. During this forward movement, the pressure inside the cylinder on the discharge end can be as high as 30,000 psi.
Apparently, the cylinder has to have the strength to take the high pressure of injection. Besides, such a cylinder has to have a good resistance to wear and corrosion by the heated plastic resin. If the bore of the cylinder is enlarged due to wear or corrosion, the plastic will leak back through the increased clearance between the screw and cylinder bore surface. As a result, the proper pressure and injection action cannot be maintained.
One approach to make such a cylinder is to make it out of a heat treated high strength steel. The inner bore surface is nitrided to provide adequate wear resistance. However, the nitrided layer is only 0.005 to 0.020 inches deep, and the concentration of nitride decreases with the depth rapidly. The wear resistance of the nitrided surface will drop quickly with time as the very hard top surface is worn. Furthermore, such a cylinder has a poor corrosion resistance to various corrosive polymers, such as fluoropolymer, and will become useless after a few months' operation.
A second approach is to cast a hard, wear resistant alloy inside a hollow steel cylinder by a centrifugal casting process. The melting point of the alloy to be cast is lower than that of the steel by a few hundred degrees Fahrenheit. In this process, the alloy is loaded inside the hollow cylinder and the ends of the cylinder are sealed. The cylinder is then placed in a furnace at a temperature high enough to melt the alloy but sufficiently lower than the melting point of the steel. The steel cylinder is then rotated rapidly about its axis to distribute the molten alloy into a continuous layer about the inside of the cylinder. Upon cooling, this molten alloy solidifies and metallurgically bonds to the steel bore to form a hard, wear resistant inlay. This inlay then is honed to the correct diameter and surface finish.
In processing various corrosive plastics, the inlay is made of nickel or cobalt alloys, which have very good corrosion resistance as well as wear resistance. The thickness of the cast inlay is about 0.060 to 0.125 inches, much thicker than that of the nitrided layer. The chemical composition, corrosion resistance, hardness, and wear resistance are uniform across the entire inlay thickness. The excellent wear and corrosion resistance of this type of inlay makes the cylinder last 5 to 10 times that of a nitrided barrel. The bimetallic cylinder has gained its popularity in the plastic industry over the years. However, it does have a weakness. The excessive heating during the casting process causes tremendous grain growth in the microstructure of the cylinder steel. Additionally, all corrosion resistant nickel or cobalt based inlay alloys require slow cooling after casting to avoid cracking because of their brittle nature and poor resistance to thermal shock. As a result, the steel typically has a very coarse pearlitic structure and has poor strength.
Facing the increasing demand of higher pressure for the injection molding process, the bimetallic cylinder often cannot hold the pressure required. The solution to this problem is to adopt a sleeve design on the discharge end, which is the high pressure end. After a bimetallic cylinder is cast, an outer steel cylinder, a sleeve, made of strong heat treated steel is put on the discharge end of the bimetallic cylinder with a shrinkage fit, i.e., the inner diameter of the sleeve is smaller than the outer diameter of the bimetallic cylinder by a few thousandths of an inch. The sleeve is put on the high pressure end of the bimetallic cylinder after the sleeve is heated to 500.degree. F. to 700.degree. F. to expand it while the bimetallic cylinder is kept at room temperature. The sleeve produces a compressive stress on the bimetallic cylinder and increases the pressure carrying capacity of the cylinder.
This design not only increases the cost of the product but also sometimes causes operational problems. At times, plastic at the discharge end can be forced into the crevice between the sleeve and the inner bimetallic cylinder during operation resulting in collapsing of the cylinder bore. The screw inside will then be seized, and the operation has to be terminated. Furthermore, the sleeve only strengthens the discharge end of the cylinder. The pressure can also build up away from the discharge end. This pressure can be significant if the machine goes through a cold start procedure. The region not protected by the sleeve then will crack due to over-pressurization.
Another approach to strengthen the bimetallic cylinder is to heat treat the bimetallic cylinder to increase the strength of the steel. However, all the existing nickel or cobalt alloys, which have good corrosion and wear resistance, will crack in any heat treating process.
Overall, the following have to be satisfied to make a strong bimetallic cylinder with good corrosion and wear resistance:
1. The steel has to be thermally compatible with the inlay. PA1 2. The inlay has to have good corrosion and wear resistance. PA1 3. The inlay has to be strong and free of defects. PA1 4. The steel has to have high strength after the centrifugal casting process.
If the steel is not thermally compatible with the inlay, the excessive thermal stress created during cooling will crack the inlay. The inlays made by centrifugal casting generally are quite strong due to their high hardness. However, they typically have microcracks in their dendritic casting structure. These microcracks can open up and propagate. If an inlay has many such microcracks of significant length, the bimetallic cylinder will have poor pressure carrying capacity.
The pressure carrying capacity of a cylinder is critically determined by the yield strength of the steel. The inlays are hard and brittle and do not allow any plastic deformation. The strain at the fracture point is about 0.15 to 0.25% Once the steel yields, the inlay will crack. Generally speaking, if the steel has a higher yield strength, the cylinder will have a higher pressure carrying capacity.